1. Field of the Invention
The present invention relates generally to receivers, and more particularly, to multi-channel receivers using automatic gain control.
2. Related Art
Many communication receivers employ Automatic Gain Control (AGC) circuits to maintain a received signal within a dynamic range of the receiver. Typically, the AGC includes a signal level detector to indicate whether an output power level of the receiver is too strong or too weak, and gain control to adjust one or more amplifiers in response to the power level, thereby establishing a proper output power level. This operation attempts to balance the detrimental but competing effects of noise and distortion.
Existing tuners typically rely on an external level detector, which is part of a larger receiver system, of which the tuner is also a part. This level detector is responsive only to the output of the tuner. At this point, all received channels (for example, TV channels) except a desired channel have been removed by one or more stages of Intermediate Frequency (IF) filtering. An output of the level detector is compared to a reference level, and from this comparison a feedback gain control signal for the tuner is derived. The gain control signal causes an increase or decrease in receiver gain, as appropriate. This feedback signal may be filtered to obtain a desired AGC system characteristic.
Some conventional tuners have two Variable Gain Amplifiers (VGAs): for example, a Radio Frequency (RF) VGA in an RF stage and an IF VGA in an IF stage following the RF stage. A single gain control signal, derived in the IF stage, feeds both VGAs. If two channels, including a desired channel and an adjacent undesired channel are present in a received signal, and the desired channel is much weaker than the adjacent channel, a conventional AGC will often allow too much gain in the RF stage, since the conventional AGC is responsive to only the desired channel level (in the IF stage). Excessive distortion will then result from the presence of the large adjacent channel. There is a need therefore for an AGC system that overcomes the above-mentioned and other related problems in conventional receivers.